1. Field of the Invention
The present invention relates to a liquid crystal display driving method and a liquid crystal display device, and more specifically to a technique suitably applied to a thin-film transistor liquid crystal display panel that performs multiple-gradation display.
2. Prior Art
An active matrix color liquid crystal display device using thin-film transistors (TFTs) has been described in publications, such as Nikkei Electronics, Nikkei McGraw-Hill, Sept. 10, 1984, pp. 211-240. TFT liquid crystal displays are used as a small, low power consumption display, primarily for the monitors in microcomputer systems. For office automation equipment, there are growing demands for display devices that can display an image of multiple gradations and multiple colors. There are multiple-color drivers that use CMOS switches for outputting gradation level (or brightness level) voltages. An example of such a driver is the HD66310T driver described in Hitachi LCD Driver Data Book, Hitachi, Ltd., Mar. 1990, pp. 650-664 (Japanese edition) and pp. 910-929 (English edition).
The driving voltages for a conventional liquid crystal display device with multiple gradations, as shown in FIG. 4, are such that gradation voltages Vsigl-Vsign are converted into alternating voltages with a reference voltage VC as their average. The reference voltage VC is also used as a white signal in the case of normally white. In pixel electrodes that act equivalently as capacitors, an interference voltage is generated at the gate electrode of the TFT transistor. This interference voltage may cause an afterimage phenomenon. To prevent this, a voltage Vcom applied to a common electrode of the TFT liquid crystal display panel is shifted from the reference voltage VC to produce a DC voltage VDC that cancels the interference voltage.